The field of the present invention is in the art of plating metal onto a substrate and more particularly that of methods employing electroplating and electroless plating techniques.
Electroplating and electroless plating are two widely used methods for plating metal onto a substrate. The choice of which method to employ for a given plating step is as often controlled as much by the choice of the substrate or disadvantages of the respective methods as vis-a-vis the other methods.
Electroplating consists of depositing an adherent metal coating on a substrate for protection purposes. The substrate to be plated is connected to one terminal of a d-c voltage source and placed in an electrolyte. The metal to be deposited is connected to the other terminal and similarly immersed in the electrolyte. The transfer of the metal is accomplished via the ions contained in the current flowing between the electrodes.
Electroless plating involves the use of a plating bath without the imposition of any electric current where the substrate is plated by reduction of the plating metal from a solution of a salt of a plating metal. The plating solution contains controlled reducing agents which are generally either catalyzed by the surface of the substrate, or by some catalytic metal placed onto the surface both to initiate the reduction and to give good adherence. Since the plated-on surface is autocatalytic, an electroless process can be used to build up good thicknesses.
In the field of electronics, microstrip circuitry is increasingly being fabricated on soft dielectric materials clad with copper used as the conductor. This packaging scheme is much cheaper than the use of ceramic substrates with either thick or thin film metallizations. However, with the use of copper as the conductor at least four problems have been encountered. First, the copper including the circuit side edges must be protected to prevent corrosion and oxidation. Second, the conductor surface must remain solderable. Third, some areas of the circuit conductor surface must be able to have components thermocompression bonded to them. Fourth, many areas of the circuit such as long transmission lines or microwave combiners are sensitive to increases in insertion loss.
Electroless tin plating is applied to the copper conductor on many microstrip circuits. Since the plating process is electroless the conductors can be plated after etching, thus the conductor sidewalls are also protected by tin plating. This technique provides a surface readily solderable with lead-tin solders while protecting the copper circuitry from oxidation and corrosion. However, some circuits have areas which must have components thermocompression bonded to them. Tin is not acceptable for this type of bonding. Even more importantly, in some circuits there are areas where insertion loss levels are critical. Tin, because of its lower conductivity than copper, causes an increase in insertion loss when plated on copper circuitry (due to skin effect).
Gold plating seems to be the one material that meets all four requirements of copper protection, solderability, bondability and high conductivity. The electroless gold plating technique would seem to be a logical choice because electrical continuity of the circuit is not needed to accomplish plating. However, electrolessly plated gold films are inferior to electroplated gold films for solderability and bondability. Also, soft dielectric microstrip circuits are built on aluminum carriers used for electrical grounding, mechanical support, and for the mounting of both active and passive components. Because of the high ph (very alkaline) of available electroless gold baths, the aluminum carrier would be rapidly attacked during the plating process.
Gold electroplating baths provide gold films of excellent bondability and solderability. However, all the metal to be plated must be electrically continuous to accomplish the plating operation. Since the plating has to be applied after etching for reasons discussed earlier, the etched circuitry must be electrically shorted together, plated and then unshorted. One method used successfully was to solder wires to the areas of the circuit that needed to be gold plated. These wires were then joined and connected to the gold plating bath cathode to accomplish the electroplating operation. After plating, the wires were removed. Areas that didn't require gold plating were then coated with solder. This technique has been found to be expensive and very time consuming.
Since many circuits are complex with many discontinuous isolated areas such as tuning pads and component mounting pads, it is feasible to gold plate only those areas of the circuit where insertion loss level is critical and where bondability is needed. All remaining areas for which continuity of the circuitry is not needed can be electrolessly tin plated. This invention accomplishes selective electroless tin plating and gold electroplating of etched circuitry while solving the time consumption problem of soldering wires to all areas of the circuit that need gold plating.